1. Field of the Invention
The present invention relates to a method of inhibiting the corrosion of low carbon steel surfaces in water-carrying systems, and to compositions for use in such a method, particularly where the water of the system is oxygen-bearing. More particularly, the present invention relates to the use of compositions comprising a combination of aminomethylphosphonic acid compounds and polymaleic anhydride or amine adducts thereof, and optionally zinc, to inhibit the corrosion of low carbon steel surfaces in water-carrying systems.
The term "aqueous", as used herein, is intended to describe water in any physical state and to include water in which is dissolved or dispersed any substance, for example, inorganic salts in brine or seawater.
Polymaleic anhydride, as used herein, is intended to include hydrolyzed polymaleic anhydride, which is essentially polymaleic acid. Under most ambient conditions, such hydrolysis to the acid form will take place.
Corrosion of low carbon steel surfaces in a water-carrying system consists of the destruction of the metal by chemical or electrochemical reaction of the metal with its immediate environment.
Where the corrosion is electrochemical in nature, a transfer or exchange of electrons is necessary for the corrosion reaction to proceed. When corrosion of the metal takes place, two partial electrochemical processes occur, and must occur, simultaneously. There is an anodic oxidation reaction in which metal ions go into solution, leaving behind electrons; and a cathodic reduction reaction in which species in solution are reduced by consuming the electrons produced by the anodic reaction. Where the water system contains oxygen, these two processes may be illustrated by the following equations: EQU Anodic oxidation: Fe.fwdarw.Fe.sup.++ +2e.sup.- EQU Cathodic reduction: 2H.sub.2 O+O.sub.2 +4e.sup.- .fwdarw.4OH.sup.-
The two ionic reaction products, ferrous ion and hydroxyl ion, combine to form ferrous hydroxide, Fe(OH).sub.2, which is then oxidized to form rust, ferric hydroxide, Fe(OH).sub.3. The principal factors influencing the corrosion process are the characteristics of the water of the system, the rate of water flow, the temperature of the system and the contact of dissimilar metals in the system. The variable characteristics of the water which determine its corrosiveness are its dissolved oxygen concentration, carbon dioxide content, pH and concentration of dissolved solids.
The presence of oxygen dissolved in the water of a system is primarily the result of contact of the water with the atmosphere. The oxygen solubility in water is temperature and pressure dependent, with an increase in pressure increasing solubility, and with an increase in temperature lowering the oxygen solubility.
Corrosion produced by the presence of oxygen in the water of a system can take place in the form of small pits or depressions besides general metal loss. As the corrosive process continues, these pits or depressions increase in area and depth and a nodule of corrosion products is formed. The corrosive attack is more severe when taking place in the form of pits or depressions since this permits deeper penetration of the metal and more rapid failure at these points.
2. Description of the Prior Art
Heretofore polymaleic anhydride and copolymers and derivatives thereof have been employed as scale inhibiting agents. See, for example, U.S. Pat. Nos. 2,723,956; 3,289,734; 3,293,152; 3,578,589; and 3,715,307. Inorganic polyphosphates have been similarly employed. See, for example, U.S. Pat. Nos. 2,358,222; 2,539,305; and 3,434,969.
A variety of compositions have been employed in the art for the purpose of inhibiting corrosion of surfaces in water-carrying systems where the cause of the corrosion is dissolved oxygen. Polyphosphates, for example sodium tripolyphosphate, are widely used in the treatment of once-through systems. See U.S. Pat. No. 2,742,369. Silicates, for example, sodium silicate, have also found acceptance.
U.S. Pat. No. 3,483,133 discloses a corrosion inhibiting composition comprising amino tris(methylenephosphonic) acid compounds in combination with water soluble zinc salts. U.S. Pat. No. 3,762,873 discloses a corrosion inhibiting method using substituted succinimides. Canadian Pat. No. 854,151 discloses a composition and method for inhibiting corrosion and/or the formation of calcium and magnesium containing scales where a combination of organophosphonic acid compounds and water soluble polymers having carboxyl or amide groups is employed. U.S. Pat. No. 3,810,834 discloses a method of treating the water of an aqueous system with hydrolyzed polymaleic anhydride having a molecular weight of 300 to 5,000 for the purpose of inhibiting scale formation; while U.S. Pat. Nos. 3,897,209; 3,963,636; and 4,089,796 disclose the use of the same hydrolyzed polymaleic anhydride material in combination with a zinc salt for the purpose of inhibiting both corrosion and scale formation.
U.S. Pat. No. 3,965,027 discloses certain amine adducts of polymaleic anhydride for use in scale inhibition and corrosion inhibition.
U.S. Pat. No. 4,105,581 discloses a corrosion inhibitor comprising a phosphate and phosphonate, which further includes a homopolymer of maleic acid or maleic anhydride.
However, none of the prior art described above in any way suggests the synergistic results obtained with the novel compositions of the present invention when used to inhibit corrosion of low carbon steel in an aqueous system.